A topological space such that for every closed subset of and every point , there is a continuous function such that and .
This is the definition given by most authors (Kelley 1955, p. 117; Willard 1970, pp. 94-95). However, some authors (e.g., Cullen 1968, p. 130) require the additional condition that be a T1-space. In any case, every completely regular space is regular, and the converse is not true.